Why does Tor Browser use Firefox?
Tor Browser is a set of design goals for anonymous browsing.
So, while the current Tor Browser that's distributed is a patched version of Firefox, this is because currently Firefox is the only browser that is close enough to the requirements to meet these design goals without extensive re-engineering or having to maintain a fork.
Firefox also provides an "Extended Support Release" which is what Tor Browser is based on, which only gets security updates, not functionality updates which reduces the frequency and workload of maintaining the patch set.
If you have the money to pay for, or happen to know a team of developers with the time to maintain, a fork of another browser such that it complies with the specification then I'm sure your contribution would be appreciated.
Remember, Tor Browsers main goal is anonymity and unlinkability. It has successfully forced Eve to become Mallory. It's likely that any attempts to use other browsers without meeting the specification above would give Eve a much easier time, making Mallory an expense that need not be spent.